The present invention relates to a method for compensating asymmetry in a reproduction signal from an optical recording medium. The invention further relates to an apparatus for reading from and/or writing to optical recording media using such method.
For high data storage densities on optical recording media, the modulation transfer function drops very steeply. The high frequency components of the analog reproduction signal are, therefore, attenuated considerably compared to the low frequency components. In case of a blu-ray disk (BD), which is currently under development, with a storage capacity of about 25 gigabytes, the shortest run-length components (2T) are attenuated by a factor of more than 20 dB compared to the longest run-length components (8T). This results in a large amount of inter-symbol interference. The eye-pattern, i.e. the high frequency signal obtained by summing the output signals of a photodetector array used in an apparatus for reading from and/or writing to optical recording media (xe2x80x9creproduction signalxe2x80x9d) is even without noise nearly closed. Furthermore, the reproduction signal is also non-linear, which results in a strong asymmetry of the eye-pattern. This can, inter alia, be caused by non-optimum recording conditions like an excessive write power, leading to different lengths of marks and spaces on the optical recording medium. The amount of this asymmetry can be larger than the amplitude of the shortest run length signal.
For a reliable data detection, the mid-level signal, which is necessary for further processing of the reproduction signal, has to be placed exactly in the middle of the shortest run-length component of the reproduction signal. This can be achieved by subtracting an offset compensation signal, which is generated by an offset compensator, from the reproduction signal.
Such a solution is, for example, disclosed in the U.S. Pat. 6,324,144. The document shows an apparatus for correcting asymmetry existing in a reproduction signal by processing the reproduction signal in digital form. An analog-to-digital converter converts the analog reproduction signal to a digital reproduction signal. A predetermined asymmetry compensation signal is then added to the digital signal to obtain an asymmetry compensated signal. Finally, a binary non-return-to-zero-inverted (NRZI) data signal is detected from the asymmetry compensated signal. This binary data signal is output by the apparatus. For determining the asymmetry compensation signal, the zero crossing points of the digital reproduction signal are detected. The zero crossing points are needed for controlling a sign bit counting operation, which is used for controlling an asymmetry corrector.
The solution disclosed in the document, which has been developed for DVD-RAM where the shortest run-length is 3T, has the disadvantage that the detection of zero crossing points cannot be reliably performed if the amplitude of the shortest run-length components is smaller than the asymmetry of the reproduction signal. In this case the shortest run-length components are nearly vanishing in the noise floor and the zero crossing points cannot easily be detected.
It is, therefore, an object of the invention to propose a method for compensating an offset in an asymmetric reproduction signal capable of compensating the offset even if the amplitude of the shortest run-length components is smaller than the asymmetry of the reproduction signal, i.e. if the detection of zero crossing points is not possible for the shortest run-length components.
According to the invention, this object is achieved by a method for compensating an offset in an asymmetric reproduction signal, whereby an offset compensation signal is subtracted from the reproduction signal, the offset compensation signal being generated by an offset compensator, comprising the steps of:
detecting a binary data signal from the asymmetric reproduction signal; and
using the binary data signal for obtaining the offset compensation signal.
Using the binary data signal, which only assumes two discrete values, for obtaining the offset compensation signal has the advantage that the offset compensation signal can be obtained with a much higher reliability compared with the digital reproduction signal, which assumes a plurality of discrete values. Even when a detection of zero crossing points is not possible, the offset compensation signal can still be obtained.
Favorably, the method further comprises the step of detecting the shortest run-length components of the binary data signal for obtaining the offset compensation signal. Since the shortest run-length components are most affected by the asymmetry of the reproduction signal, it is sufficient to use only these components for obtaining the offset compensation signal. In this case, every time a shortest run-length component is detected, an enable signal is generated for enabling the offset compensation. Of course, it is also possible to detect signal components with another run-length and to generate the respective enable signal. A secure run-length detection based on the digital reproduction signal as as known from prior art, i.e. before detecting the binary data signal, is not possible if the amplitude of the shortest run-length components is smaller than the asymmetry of the reproduction signal.
Advantageously, the method further comprises the step of delaying the asymmetric reproduction signal before obtaining the offset compensation signal and/or before subtracting the offset compensation signal from the reproduction signal. This allows to compensate for the processing delay caused by the detection of the binary data signal from the asymmetric reproduction signal and by the run-length detection, so that the enable signal, and correspondingly the asymmetry compensation signal, coincides exactly with the shortest run-length samples of the reproduction signal. Delaying the asymmetric reproduction signal can, for example, be performed by a register chain.
Favorably, the method further comprises the step of centering the asymmetric reproduction signal with regard to a digital zero before detecting the binary data signal. This centering is, for example, performed by passing the reproduction signal through a slicer. Centering the asymmetric reproduction signal without compensating the offset is sufficient for a reliable run-length detection until the offset compensation has settled to a final offset compensation signal.
Advantageously, a partial response maximum likelihood detector or a bit-by-bit detector is used for detecting the binary data signal. Both detectors deliver a non-return-to-zero (NRZ) data stream at their output, which can be used for obtaining the asymmetry compensation signal. While the partial response maximum likelihood detector, e.g. a partial response equalizer in combination with a Viterbi detector, delivers a lower bit error rate and has a higher performance, the bit-by-bit detector is less expensive and simplifies the necessary delay of the reproduction signal samples.
According to a further refinement of the invention a plurality of run-lengths of the binary data signal are detected for obtaining run-length dependent offset compensation signals and enabling the offset compensation accordingly. For each signal sample the offset compensation signal corresponding to the run-length of the signal sample is used for offset compensation. In this way, the offset is not only compensated for the shortest run length, but selectively also for other run-lengths, which are allowed by the channel modulation, leading to an even more reliable data detection and hence to a lower bit error rate. In this case, it is perfectly possible to use a partial response maximum likelihood detector for detecting the binary data signal used for obtaining the different offset compensation signals, and to use a simple bit-by-bit detector for controlling a multiplexer used for selecting the appropriate offset compensation signal.
According to another aspect of the invention an offset compensator for compensating an offset in an asymmetric reproduction signal, the offset compensator comprising an offset compensation signal generator for generating an offset compensation signal and a subtractor for subtracting the offset compensation signal from the reproduction signal, further comprises a binary data signal detector for generating a binary data signal from the asymmetric reproduction signal, whereby the binary data signal is used for obtaining the offset compensation signal.
Such an offset compensator has the advantage that it works much more reliable than an offset compensator using the digital reproduction signal for obtaining the offset compensation signal.
Favorably, the offset compensator further comprises a shortest run-length detector for detecting the shortest run-length components of the binary data signal for obtaining the offset compensation signal. The shortest run-length detector will generate an enable signal for enabling the offset compensation every time a shortest run-length component is detected. In addition, a detector for other run-length components can also be advantageously provided.
Advantageously, the offset compensator further comprises delay means for delaying the asymmetric reproduction signal before obtaining the offset compensation signal and/or before subtracting the offset compensation signal from the reproduction signal. In this way possible delays caused by signal processing in the binary data signal detector and/or in the shortest run-length detector are taken into account, so that the enable signal, and correspondingly the asymmetry compensation signal, coincides exactly with the shortest run-length samples of the reproduction signal. A register chain can, for example, be used as a delay means. Of course, other delay means can also be used. In case another run-length is detected, corresponding delay means can also be advantageously provided.
Favorably, the offset compensator comprises means for centering the asymmetric reproduction signal with regard to the digital zero before generating the binary data signal. A possible centering means is a slicer, which even without compensating the offset centers the reproduction sufficiently for a reliable run-length detection until the offset compensation has settled to a final offset compensation signal.
According to one embodiment of the invention the offset compensator preferably uses a partial response maximum likelihood detector or a bit-by-bit detector for generating the binary data signal. While the partial response maximum likelihood detector, e.g. a partial response equalizer in combination with a Viterbi detector, delivers a lower bit error rate and has a higher performance, the bit-by-bit detector is less expensive and simplifies the necessary delay of the reproduction signal samples. Of course, other type of binary data signal detectors can also be used.
Favorably, a plurality of run-length detectors is used for detecting a plurality of run-length of the binary data signal for obtaining run-length dependent offset compensation signals and for enabling the offset compensation accordingly. In this way the offset can be compensated for any desired run-length, leading to an even more reliable data detection.
According to the invention, an apparatus for reading from and/or writing to recording media performs a method or comprises an offset compensator according to the invention. Such an apparatus has the advantage that it allows a more reliable data detection and hence a lower bit error rate upon reproduction of high-density optical recording media.